StreamOfTheSky
Adventurer
Seriously. Look around you. Count how many shadows there are other than your own.
I don't know...it always seemed kind of limited, to me. Sure, there may be scattered shadows in different places. But if you just go into one and stay there, the enemy knows where you are roughly. If you try to hop between shadowy areas, enemies see you do so (I think C.Adv had rules for jumping from points of cover, though).
On the other hand, a LOT of common game areas are "natural" terrain. Most dungeons even, are cave complexes. If you're on another plane but outdoors, that's still "natural terrain" for that plane. Only when you're actually in a building or "man/other-made structure or paved town/city roads is it typicaly no longer "natural."
But I don't know how much this even matters to someone looking for ways to more efficiently kill dragons, anyway. Its not like he has sneak attack dice riding on being hidden. Useful life skill, sure. Not terribly pertinent for what the OP asked for, though.